The present invention relates to a method and an apparatus for treating salt water of a high salinity by passing it through a plurality of high and low pressure reverse osmosis membrane modules.
Since coal mines and water veins coexist with one another and the water vein is thereby able to pass through a rock salt layer, in, for example, eastern Europe, water discharged from the coal mine shows excessively high levels of salinity of 30,000 to 80,000 ppm. Therefore, the flow of this salt water into rivers, lakes and marshes causes serious environmental contamination in certain regions.
In order to overcome the above-described problem while preventing a secondary contamination, it is necessary to completely treat salt water in such a manner that it is separated into potable water or industrial water and concentrated water containing salt at high density or solid salt while preventing the discharge of salt water into rivers, lakes and marshes.
In order to achieve the above-described object, it might be considered feasible to employ a sole evaporation method, a method resulting from the combination of a single-stage reverse osmosis membrane method and the evaporation method and a method resulting from the combination of an electro-dialysis method and the evaporation method.
On the other hand, an apparatus capable of obtaining fresh water from sea water has been disclosed in Japanese Patent Laid-Open No. 55-31459. According to this disclosure, sea water, the salinity level of which is about 35,000 ppm, is passed through a high pressure reverse osmosis membrane module so as to supply desalted water to a low pressure reverse osmosis membrane module.
Another apparatus has been disclosed in Japanese Patent Utility Model Laid-Open No. 58-108195 in which a reverse osmosis membrane module displaying a low salt rejection and a reverse osmosis membrane module exhibiting a high salt rejection ratio are connected in series to each other in one vessel in such a manner that the reverse osmosis membrane module exhibiting a high salt rejection ratio is disposed in the lower stream in the apparatus.
In the case where the sole evaporation method is employed, the overall portion of potable water is obtained from evaporation and condensation, causing an excessively large energy loss to take place (since the phase change is involved according to the evaporation method, energy costs cannot be reduced in comparison to the fresh water generating method by means of the reverse osmosis membrane method or the like).
In the case where the single-stage reverse osmosis membrane method and the evaporation method are combined, the energy reduction can be achieved relatively satisfactorily with respect to the above-described sole evaporation method. However, the degree of concentration which can be realized by the single-stage reverse osmosis membrane is unsatisfactory, causing the necessity of evaporating a large quantity of water. Therefore, the energy consumption cannot be reduced satisfactorily.
Furthermore, if the salinity level of salt water supplied to the reverse osmosis membrane exceeds 60,000 ppm, ordinary reverse osmosis membranes for generating fresh water from sea water cannot be employed because of the restriction caused from the osmotic pressure (because the pressure is raised excessively). On the other hand, the low pressure reverse osmosis membrane cannot generate desalted water of a potable water quality.
In the case where the electrodialysis method and the evaporation method are combined, an-excessive amount of energy is consumed in the electrodialysis step in a quantity in proportion to the salinity. Although the electrodialysis can be employed for the concentration step, it is not preferable because a large quantity of medium salinity water is undesirably discharged from the desalting side of the electrodialysis apparatus.
The system disclosed in Japanese Patent Laid-Open No. 55-31459 is a system for obtaining fresh water from sea water. Therefore, the degree of concentration which can be achieved at the time of concentrating water is relatively low compared to that which may be obtained by the present invention. What is even worse, concentrated water is abandoned.
The system disclosed in Japanese Utility Model Laid-Open No. 58-108195 is arranged in such a manner that salt water, the pressure of which has been raised to a predetermined level, is sequentially passed through a plurality of membrane modules. However, the above-made disclosure does not contain a technological concept of reducing the pressure of the medium density concentrated water to be supplied to the low pressure membrane module disposed at the second stage and a concept of raising the pressure of coarsely desalted water to be supplied to the high pressure membrane module disposed at the second stage.